A prolyl endopeptidase inhibitor was isolated from the ethyl acetate soluble fraction of Phyllanthus ussurensis. The active compound was identified as an ellagitannin, corilagin. It was shown to non-competitively inhibit prolyl endopeptidase (PEP) with the $IC_{50}$ value of $1.17 imes $10^{-6}mu$M. The Ki value was $6.70 imes 10^{-7}$ M. Corilagin was less inhibitory to other serine proteases such as chymotrypsin, trypsin, and elastase, indicating that it was relatively a specific inhibitor of PEP. Corilagin also effectively inhibited reactive oxygen species such as hydroxide and superoxide anion radical, hydrogen peroxide, and DPPH. Especially, corilagin showed potent scavel1ging activity on the superoxide anion radical in the ESR method ($IC_{50} =3.79 imes 10^{-6}$M) as well as xanthine oxidase system.